The present invention relates generally to constructed wetlands for remediation of groundwater contamination, primarily chlorinated ethane solvents, and particularly to improvements for a upward flow constructed wetland cell.
In a 2007 journal article titled “Development of a Wetland Constructed for the Treatment of Groundwater Contaminated by Chlorinated Ethenes,” James P. Amon, Abinash Agrawal, Michael L. Shelley, Bryan C. Opperman, Michael P. Enright, Nathan D. Clemmer, Thomas Slusser, Jason Lach, Teresa Sobolewski, William Gruner and Andrew C. Entingh, Ecological Engineering, Vol. 30, pp. 1-6, 2007, some of the co-inventors of the present invention, and others, described a novel upward flowing constructed wetland for removing containments from groundwater. That journal article, and all journal articles and other papers referenced in this description, are fully incorporated by reference into this description. Parts of this description may include passages from those papers, with grateful thanks to those authors, but often without specific attribution.
Chlorinated solvents are a major source of groundwater pollution in the United States and pose significant health risks where groundwater is the source of drinking water. Compounds such as perchloroethylene (PCE) and trichloroethylene (TCE) were produced in large quantities after World War II and often leaked from underground storage tanks or were disposed of improperly. Where PCE is present, TCE, isomers of dichloroethylene (DCE), and vinyl chloride (VC) are often present as daughter products of its in situ degradation. These low molecular weight chlorinated hydrocarbons are relatively insoluble and much denser than water and, as such, they readily penetrate water-saturated soils and form a pool at the bottom of an aquifer. This pool and the soil zone above it become a constant source of contaminant when groundwater moves through it and creates a plume of contamination. TCE is the most common groundwater contaminant and a major contaminant at many military bases around the country.
It had been earlier discovered that contaminants such as TCE could be broken down by naturally occurring microorganisms, triggering research on ways to utilize these organisms in bioremediation treatment systems.
A key to successful adaptation of those natural bioremediation treatment systems was that degradation of chlorinated ethenes may occur in both anaerobic and aerobic environments, but PCE can be converted to TCE only under anaerobic conditions, while chlorinated ethenes can be chemically and microbially converted to progressively less chlorinated ethenes anaerobically by reductive dechlorination.
Methane as a growth substrate has been successfully studied in a number of systems. During co-metabolic degradation of TCE with methane, the methane-oxidizing bacteria (methanotrophs) produce a non-specific enzyme, methane monooxygenase (MMO), which oxidizes methane as its substrate and can also fortuitously degrade TCE.
The 2007 journal article generally described a new upward flow constructed wetland cell for treating groundwater contaminated with chlorinated aliphatics. Although wetlands have been used for water treatment for many years, the unique concept of the new constructed wetland was upward vertical water flow through individually designed soil layers, or zones, within the constructed wetland cell to provide the required sequential conditions to sequentially alter the contaminants to produce non-hazardous by-products.
Briefly, the new upward flow constructed wetland fed contaminated ground water sequentially through, from bottom to top:
(1) a first layer of organic soil where PCE would undergo anaerobic dechlorination to TCE, DCE and VC, with methane formation.
(2) a second layer of iron rich soil where TCE, DCE and VC would undergo iron reduction degradation producing TCE+, DCE+ and VC with CO2 formation.
(3) a third layer where wetland plant root zones create an oxygenated zone where methane-oxidizing microorganisms oxidize the methane and, thanks to the threshold presence of the methane, also advantageously co-metabolize and degrade the TEC, DEC and VC.
Despite the success of the upward flow constructed wetland cell, its performance can still be improved.